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Using C/C++ and Fortran together:

This tutorial covers mixing C/C++ and FORTRAN together, allowing C/C++ to call FORTRAN functions and FORTRAN to call C/C++ functions. Integrating C/C++ and FORTRAN into a single executable relies upon knowing how to interface the function calls, argument list and global data structures so the symbols match in the object code during linking. FORTRAN and C/C++ interoperability is sucessful when datatypes are matched appropriately, arguments passed correctly, the code linked properly and the executable runs without error.

Related YoLinux Tutorials:

°FORTRAN Compilers, Linux Software development

°C++ Info, links

°Emacs and C/C++

°Advanced VI

°Clearcase Commands

°Google C++ Unit Test

°Jenkins CI

°Jenkins Plugins for C++

°YoLinux Tutorials Index




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Comparison of FORTRAN and C/C++ datatypes:

FORTRAN C/C++
byte unsigned char
integer*2 short int
integer long int or int
integer iabc(2,3) int iabc[3][2];
logical long int or int
logical*1 bool
(C++, One byte)
real float
real*8 double
real*16 long double
complex struct{float realnum; float imagnum;}
double complex struct{double dr; double di;}
character*6 abc char abc[6];
character*6 abc(4) char abc[4][6];
parameter #define PARAMETER value

This table shows the appropriate types to use for FORTRAN and C/C++ interoperability.


FORTRAN and C Arrays:

Order of multi dimensional arrays in C/C++ is the opposite of FORTRAN.

Native FORTRAN layout (collumn-major order): INTEGER A(2,3)
a(1,1)a(2,1)a(1,2)a(2,2)a(1,3)a(2,3)
Or INTEGER A(2,2,2)
a(1,1,1)a(2,1,1)a(1,2,1)a(2,2,1)a(2,2,1)a(1,1,2)a(2,1,2a(1,2,2)a(2,2,2)
Native C layout (row-major order) is NOT equivalent to the FORTRAN layout: int a[2][3];
a[0][0]a[0][1]a[0][2]a[1][0]a[1][1]a[1][2]

Thus:

  • Equivalent multidimension arrays:
    • FORTRAN 77: INTEGER I(2,3,4)
    • FORTRAN 90: INTEGER, DIMENSION(2,3,4) :: I
    • C: int i[4][3][2];
  • Accessing a FORTRAN array in C:
    Native FORTRAN:
       INTEGER   I,J,NI,NJ
       PARAMETER(NI=2,NJ=3)
       INTEGER   B(NI,NJ)                same as B(2,3)
    
       DO J = 1, NJ                      loop to 3
           DO I = 1, NI                  loop to 2
               PRINT 10, I, J, B(I,J)
    10         FORMAT(‘B(', I1, ',', I1, ') =', I2)
           END DO
       END DO
                  

    Native C:
        int a[3][2])
        int i, j;
     
        for (i = 0; i < 3; i++)
            for (j = 0; j < 2; j++)
                printf(“a[%d][%d] = %d\n", i, j, a[i][j]);
                  
    C array a[3][2] memory layout:
    a[0][0]a[0][1]a[1][0]a[1][1]a[2][0]a[2][1]
  • In FORTRAN 90, also check out the "RESHAPE" directive.
  • It is best not to re-dimension multi dimensional arrays within a function. Pass array size "n" and declare array as x[n][];


Linking FORTRAN And C Subroutines:

Fortran subroutines are the equivalent of "C" functions returning "(void)".

Note: The entry point names for some FORTRAN compilers have an underscore appended to the name. This is also true for common block/structure names as shown above.

FORTRAN C
CALL SUBRA( ... ) subra_( ... )

The f77 comiler flags "-fno-underscore" and "-fno-second-underscore" will alter the default naming in the object code and thus affect linking. One may view the object file with the command nm (i.e.: nm file.o).

Note: The case in FORTRAN is NOT preserved and is represented in lower case in the object file. The g77 compiler option "-fsource-case-lower" is default. GNU g77 FORTRAN can be case sensitive with the compile option "-fsource-case-preserve".

NOTE: When debugging with GDB, the Fortran subroutines must be referenced with names as they appear in the symbol table. This is the same as the "C" representation. Thus when setting a break point at the Fortran subroutine subra(), issue the comand "break subra_".

Man pages:

  • nm - list symbols from object files
  • g77/f77


Function Arguments:

All arguments in FORTRAN are passed by reference and not by value. Thus C must pass FORTRAN arguments as a pointer.

FORTRAN C
call subra( i, x) subra_( int *i, float *x)

Character variables:

  • Linux and GNU compilers: When passing character strings, the length must follow as separate arguments which are passed by value.

    FORTRAN C
    call subra( string_a, string_b) len_a = strlen(string_a);
    len_b = strlen(string_a);
    subra_( char *string_a, len_a, char *string_b, len_b)

    I have also seen the passing of a data structure containing two elements, the character string and an integer storing the length. This is common with databases such as Oracle.

  • Classic AT&T UNIX:
    When passing character strings, the length must be appended as separate arguments which are passed by value.

    FORTRAN C
    call subra( string_a, string_b) subra_( char *string_a, char *string_b, len_a, len_b)

C expects strings to be null-terminated. Thus character strings from FORTRAN which are passed back to "C" should be null terminated with CHAR(0)

CHARACTER(LEN=32) :: sample_string = "This is a sample"//CHAR(0)

or

CALL SUBRA ('A string'//CHAR(0))
              


Alternate Returns:

FORTRAN C
call sub(a,b,c,*,*)
return 1
end
int sub_(int *a,int *b,int *c)
{
return(1)
}
goto(1, 2, 3), sub()
if( sub() ) goto ERRS;

Note: When using alternate returns to turn on/off an intlevel by returning a 1/0 you must use a FORTRAN wrapper to perform this function on the CSC Norwich mainframe. This is because the C compiler is old.

  • Buffering output: Your machine may be configured where the output buffering defaults for FORTRAN and C may be configured the same or differently. C output may be buffered buffered while FORTRAN may not. If so, execute a function initialization task to unbuffer C otherwise print statements for C will be output out of order and at the end.
              #include <stdio.h>

    void ersetb(void){
    setbuf(stdout,NULL); /* set output to unbuffered */
    }


    Common Blocks:

    Fortran common block and global C/C++ extern structs of same name are equivalent. Never use un-named common blocks! Reference variables in same order, same type and with the same name for both C and FORTRAN. Character data is aligned on word boundaries.

              FORTRAN:
                   DOUBLE PRECISION X
                   INTEGER A, B, C
                   COMMON/ABC/ X, A, B, C
     
              C:
                   extern struct{
                       double x;
                       int a, b, c;
                   } abc_;
     
              C++:
                 extern "C" {
                   extern struct{
                       double x;
                       int a, b, c;
                   } abc_;
                 }
     
                  

    Note: use of extern requires that the common block be referenced first by FORTRAN. If referenced first by C then drop the extern. The extern statement states that it is trying to reference memory which has already been set aside elsewhere.

    [Potential Pitfall]: Byte alignment can be a source of data corruption if memory boundaries between FORTRAN and C/C++ are different. Each language may also align structure data differently. One must preserver the alignment of memory between the C/C++ "struct" and FORTRAN "common block" by ordering the variables in the exact same order and exactly matching the size of each variable. It is best to order the variables from the largest word size down to the smallest. Start with "double" followed by "float" and "int". Bool and byte aligned data should be listed last.

              FORTRAN:
                   INTEGER A, B, C
                   DOUBLE PRECISION D, E, F
                   LOGICAL*1  FLAG
                   COMMON/ABC/ A, D, FLAG, B, E
     
              C:
                   extern struct{
                       int a;
                       double d;
                       bool flag;
                       int b;
                       double e;
                   } abc_;
     
              C++:
                 extern "C" {
                   extern struct{
                       int a;
                       double d;
                       bool flag;
                       int b;
                       double e;
                   } abc_;
                 }
                  

    Using GDB to examine alignment:

    • Set a breakpoint in the C/C++ section of code which has visibility to the struct.
    • While in a C/C++ section of code:
      (gdb) print &abc_.b
      $3 = (int *) 0x5013e8
    • Set a breakpoint in the FORTRAN section of code which has visibility to the common block.
    • While in a FORTRAN section of code:
      (gdb) print &b
      $2 = (PTR TO -> ( integer )) 0x5013e8
    This will print the hex memory address of the variable as C/C++ and FORTRAN view the variable. The hex address should be the same for both. If not, the data will be passed improperly.

    Forcing alignment with compiler arguments: Mixing Intel FORTRAN compiler and GNU g++: use the following compiler flags to force a common memory alignment and padding to achieve a common double word alignment of variables:

    • Intel FORTRAN: -warn alignments -align all -align rec8byte
    • Intel C/C++: -Zp8
    • GNU g++: -Wpadded -Wpacked -malign-double -mpreferred-stack-boundary=8
      Example warning: warning: padding struct size to alignment boundary
    • GNU g77: -malign-double


    Example:

    FORTRAN program calling a C function:

    testF.f testC.c
          program test
    
          integer ii, jj, kk
          common/ijk/ ii, jj, kk
          real*8  ff
          character*32 cc
    
          ii = 2
          jj = 3
          kk = 4
          ff = 9.0567
          cc = 'Example of a character string'
    
          write(6,10) ii, ff
    10    format('ii= ',i2,' ff= ',f10.4)
    
          call abc(ii)
    
          write(6,20) ii
    20    format('ii= ',i2)
    
          write(6,30) ii, jj, kk
    
          call doubleIJK(cc)
    
          write(6,30) ii, jj, kk
    30    format('ii= ',i2,' jj= ', i2, ' kk= ', i2)
    
          write(6, 40) cc
    40    format(a32)
    
          stop
          end
    
          subroutine abc(jj)
          jj = jj * 2
          return
          end
    
    #include <stdio.h>
    
    extern struct
    {
       int ii, jj, kk;
    } ijk_;
    
    int doubleijk_(char *cc, int ll)
    {
       cc[ll--] = '\0';  // NULL terminate the string
    
       printf("From doubleIJK: %s\n",cc);
    
       ijk_.ii *=2;
       ijk_.jj *=2;
       ijk_.kk *=2;
    
       return(1);
    }
    

    Compile:

    • f77 -c testF.f
    • gcc -c testC.c
    • f77 -o test testF.o testC.o

    Note: If there is use of C/C++ standard libraries you may have to include the following linking arguments: -lc or -lstdc++

    Run: ./test

    ii=  2 ff=     9.0567
    ii= 4
    ii= 4 jj= 3 kk= 4
    From doubleIJK: Example of a character string
    ii= 8 jj= 6 kk= 8
    Example of a character string

    [Potential Pitfall]: If one does not terminate the string with NULL, the default with the new compilers (fort77 1.15 and gcc 4.5.2) and linker is to terminate the string with '\004' which is EOT (end of transmission). Previously this was not required but it is now. This code (cc[ll--] = '\0';) replaces the string terminating EOT character with a NULL.


    C++ calling a FORTRAN function:

    testC.cpp testF.f
    #include <iostream>

    using namespace std;

    extern"C" {
    void fortfunc_(int *ii, float *ff);
    }

    main()
    {

    int ii=5;
    float ff=5.5;

    fortfunc_(&ii, &ff);

    return 0;
    }
          subroutine fortfunc(ii,ff)
    integer ii
    real*4 ff

    write(6,100) ii, ff
    100 format('ii=',i2,' ff=',f6.3)

    return
    end

    Compile:

    • f77 -c testF.f
    • g++ -c testC.cpp
    • g++ -o test testF.o testC.o -lg2c

    Run: ./test

    ii= 5 ff= 5.500


    File I/O:

    FORTRAN unit numbers can not be shared with "C" and "C" file pointers can not be used by FORTRAN. Pass data to a function (pick one language) which does the I/O for you. Use this function by both languages.


    VAX Extensions:

    The GNU FORTRAN compilers have only ported a small subset of VAX extensions. The vast majority will require a clever re-write.

    VAX Variable Format Expressions:

    VAX expression Ported to GNU FORTRAN
          integer*4 ivar(3), nfor
    nfor=3
    ...

    write(6,100) (ivar(i), i=1,nfor)
    100 format(<nfor>I3)
          do 20 i=1,nfor
    write(6,200) ivar(i)
    200 format(I3,$) !! Supress carriage return
    20 continue

    write(6,400) !! Write carriage return
    400 format()


    VAX intrinsic functions:

    Many are not supported in GNU FORTRAN and require the creation of an equivalent library written in "C".

    Return type VAX FORTRAN intrinsic function Argument type
    Integer*4 NINT(arg)
    JNINT(arg)
    Real*4
    Integer*4 IDNINT(arg)
    JIDNINT(arg)
    Real*8
    Integer*2 ININT(arg) Real*4
    Integer*8 KNINT(arg) Real*4
    Integer*2 IIDNNT(arg) Real*8
    Integer*8 KIDNNT(arg) Real*8
    Integer*2 IIQNNT(arg) Real*16
    Integer*4 IQNINT(arg)
    JIQNNT(arg)
    Real*16
    Integer*8 KIQNNT(arg) Real*16

    Example: inint.c

    short int inint_(float *rval)
    {
    if(*rval < 0.0)
    return (*rval - 0.5);
    else
    return (*rval + 0.5);
    }

    • gcc -c inint.c
    • gcc -c idnint.c
    • ...
    • Create library: ar -cvq libvax.a inint.o idnint.o ...


    C++:

    When mixing Fortran with C++, name mangling must be prevented.

              #ifdef __cplusplus
                 extern"C" {
              #endif
              .
              .
              place declarations here
              .
              .
              #ifdef __cplusplus
              }
              #endif
            
    For more on C++ name mangling, see the YoLinux.com C++ name mangling discussion.


    The Intel FORTRAN compiler:

    The Intel FORTRAN compiler has become a popular FORTRAN compiler for Linux as it supports many of the FORTRAN extensions supported by the Compaq (old DEC vax) and SGI FORTRAN compilers. (i.e. "structure", "record", "external", "encode", "decode", "find", "virtual", "pointer", "union", various intr8insic functions, I/O directives, ...) It links with object code generated by GNU gcc/g++ compilers as well as their own Intel C/C++ compilers.

    Installation: (as root)

    • mkdir /opt/intel
    • cd /opt/intel
    • move Intel Fortram compiler tar ball to this directory.
    • tar xzf 1_fc_c_9.0.033_ia32.tar.gz
    • cd 1_fc_c_9.0.033/
    • ./install.sh
      1 : Install
      2 : provide name of an existing license file.
      License file path :
      /path-to-license-file/commercial_for_1_F2WC-5FDZV87R.lic
      (file copied to /opt/intel/licenses)
      1 (typical installation)
      Type "accept" to agree to license terms.
      Accept default location: /opt/intel/fc/9.0
      Accept default location: /opt/intel/idb/9.0
      x : Installation done

    Compiler use and user configurations:

    File: $HOME/.bashrc

    ..
    ...

    #
    # Intel Compiler
    #

    # FlexLM license server
    export INTEL_LICENSE_FILE=28518@license-server # Support for Intel FORTRAN compiler if [ -f /opt/intel/fc/9.0/bin/ifortvars.sh ]; then source /opt/intel/fc/9.0/bin/ifortvars.sh fi # Support for Intel C/C++ compiler if [ -f /opt/intel/cc/9.0/bin/iccvars.sh ]; then source /opt/intel/cc/9.0/bin/iccvars.sh fi # Support for Intel Debugger if [ -f /opt/intel/cc/9.0/bin/idbvars.sh ]; then source /opt/intel/cc/9.0/bin/idbvars.sh fi ... ... export LD_LIBRARY_PATH export PATH

    File: Makefile (snipet)

    F77=/opt/intel/fc/9.0/bin/ifort
    F77FLAGS= -extend_source -fpp -f77rtl -intconstant -ftz -pad-source -sox \
    -lowercase -warn alignments -cxxlibgcc
    CXX=g++
    CPPFLAGS=-Wpadded -Wpacked
    LDFLAGS=-L/opt/intel/fc/9.0/lib -lifport -lifcore -limf -Wabi -Wcast-align
    DEBUG=-g

    OBJS=file1.o file2.o cpp-exe: $(OBJS)
    $(CC) $(LDFLAGS) -o name-of-exe $(OBJS)

    .cpp.o:
    $(CXX) -c $(DEBUG) $(CPPFLAGS) $<

    .f.o:
    $(F77) -c $(DEBUG) $(F77FLAGS) $<
    Intel compiler directives:
    Directive Description
    -extend_source Length of line in source file allows for greater than 72 characters.
    -vax VAX FORTRAN runtime behavior. Changes read behavior. I never use this.
    -f77rtl FORTRAN 77 runtime behavior.
    -fpp Run preprocessor. i.e. handles #define and #ifdef macros.
    -intconstant Use FORTRAN 77 semantics to determine the kind of parameter for integer constants.
    -ftz Flushes denormal results to zero.
    -pad-source Specifies that fixed-form source records shorter than the statement field width should be padded with spacs (on the right) to the end of the field.
    -lowercase All function names are represented as lower case symbols.
    -sox Store compiler options and version in the executable.
    -warn alignments Warning if COMMON block records require padding for alignment.
    -align all Will get rid of warning: "Because of COMMON, the alignment of object is inconsistent with its type [variable-name]
    This requires a matching alignment for all code which links with this, C, C++ or FORTAN.

    Use GNU g++ or Intel C++ compiler to compile and link with main():
    g++ -o name-of-exe main_prog.cpp file1.o file2.o -L/opt/intel/fc/9.0/lib -lifport -lifcore -limf -Wabi -Wcast-align

    Using ddd as a front-end for the Intel debugger:

    ddd --debugger "/opt/intel/idb/9.0/bin/idb -gdb" exe-file

    [Potential Pitfall]: I found that I could not install the Intel FORTRAN compiler from an NFS mounted drive. I had to copy the Intel installation files to a local drive and install from there.

    Documentation: /opt/intel/cc/9.0/doc/main_for/mergedProjects/bldaps_for/


    Links:


    Books:

    "Introduction to Programming with Fortran"
    with coverage of Fortran 90, 95, 2003 and 77
    by Ian Chivers, Jane Sleightholme
    Springer; 1st edition, ISBN# 1846280532

    Amazon.com
    "FORTRAN 90 for Scientists and Engineers"
    by Brian Hahn
    Butterworth-Heinemann, ISBN# 0340600349

    Amazon.com
    "Introduction to FORTRAN 90 for Engineers and Scientists"
    by Larry R. Nyhoff, Sanford Leestma
    Prentice Hall; 1st edition, ISBN# 0135052157

    Amazon.com
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    Copyright © 2001 - 2011 by Greg Ippolito